The present invention relates generally to fiber optic communications systems. In particular, this invention relates to laser drive circuits within fiber optic communications systems, and more particularly to a system and method of linearizing the output power of a laser drive circuit.
In fiber-optic communications systems, multiple channels of optical data, all at different colors of the spectrum, are transmitted from a laser diode over a single fiber and input into an amplifier. The amplifiers then amplify the optical data received and retransmit the optical data. However, problems arise when there are power fluctuations on the optical data channels input into the amplifiers. If the aggregate power input into the amplifier is not linear or held at a constant level, the slope of the amplifier""s gain may change and/or the amplifier noise may increase causing optical data to be lost.
A first method for linearizing the output power of a laser diode is to use a rear-beam detector (RBD) packaged in with a laser. The output from the RBD is input into a control loop with what ever is driving the laser. The control loop then linearizes the output power from the laser diode.
The problem with this first method is that it is very difficult to design a laser diode packaged in with a RBD. The process is very labor intensive and it is difficult to control the RBD value. Three different vendors may manufacture three different laser diodes packaged with RBDs containing RBD values which significantly differ from the other. RBDs from different vendors may be used if the power output from the laser diode fluctuates over a narrow dynamic range. However, RBDs from different vendors will not work if the laser diode output power fluctuates over a wide range. To work over a wide range, the manufacturers would have to ensure that all the RBD values are the same. This would double or triple the cost of the packaged laser diodes and RBDs.
A second method of keeping the power level of the optical data channels constant is to monitor the aggregate power input into the amplifier and to dedicate various channels on the fiber to control the aggregate power level. This may be done by setting up a first order linear control loop using a MOSFET as a constant current source and modulating its bias so that the current to the diode may be raised or lowered. As the current is raised or lowered, so is the power of the dedicated channel to ensure that the aggregate power of the optical data channels remains constant.
One problem with this second method is that the power changes on these dedicated optical data channels may fluctuate veryrapidly. The power circuits located on each optical data channel may not be able to compensate for the fast power fluctuations since they work over a very narrow dynamic range. Furthermore, if all the data channels are simultaneously flipped on or off, the power circuits on each data channel cannot compensate for the huge power surge or power loss. The end result is that the power output from the laser and input into the amplifier is non-linear, thus optical data may be dropped.
Another problem with this second method occurs if an optical data receiver cannot determine what color light it is receiving. To determine what color of light the optical data receiver is receiving, a tone at a specified frequency must be passed along with the optical data to each optical data receiver. However, the first order control loop cannot simultaneously pass a tone and keep the aggregate power level constant, so it drops the tone.
The present invention provides an ultra-linear laser drive circuit that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods used for linearizing laser drive circuits.
More specifically, the present invention provides a system for linearizing and reducing the noise from the output of the laser drive circuit. The ultra-linear laser drive circuit of the present invention includes a transconductance operational amplifier circuit integrated with a low-pass filter. The transconductance operational amplifier circuit linearizes the output power of the laser drive circuit. The low-pass filter is a second-order low-pass filter which reduces the noise of the laser drive circuit.
The present invention provides an important technical advantage by providing a system and method for linearizing the output power of the laser drive circuit.
The present invention provides another technical advantage by providing a system and method for reducing the noise from the output power of the laser drive circuit by 30 to 60 dB.
The present invention provides yet another technical advantage by providing an ultra-linear laser drive circuit which does not require a rear-beam detector.
The present invention provides yet another technical advantage by providing an ultra-linear laser drive circuit which is capable of passing a tone.
The present invention provides yet another technical advantage by providing an ultra-linear laser drive circuit which reduces ground bounce.
The present invention provides yet another technical advantage by providing an ultra-linear laser drive circuit which yields an output power which is constant across many different laser diode types and brands.